The influence of interior mantle temperature on the structure of plumes: Heads for Venus, tails for the Earth

{[}1] To form mantle plumes with large heads and narrow trailing conduits requires large (> 10(2)) viscosity variations within the hot thermal boundary layer at the plume source. However, new and published data from laboratory experiments show that if a layer of fluid with a temperature-dependent viscosity is heated from below and cooled from above, convection occurs beneath a stagnant lid, and the interior fluid temperature remains close to that of the hot boundary. Consequently, the viscosity reduction across the hot thermal boundary layer is always